1. Field of the Invention
The present invention relates to a photo SI (static induction) thyristor driving circuit and its protection circuit, and more particularly to a photo SI thyristor driving circuit which can be used as a photo relay capable of controlling the flow of a large amount of current and having a high voltage resisting characteristic and to a protection circuit for preventing the photo SI thyristor from being thermally broken when the turning-on current of the photo SI thyristor flows excessively.
2. Description of the Related Art
A photo SI (static induction) thyristor is typically used as a photo relay because it has a high voltage resisting characteristic and can control the flow of a large amount of current.
FIG. 15 is a prior art example of a photo SI thyristor driving circuit, in which photo SI thyristors are indicated by numerals 1a and 1b. A resistor Ra is connected between a gate and a cathode of the photo SI thyristor 1a, and a resistor Rb is connected between a gate and a cathode of the photo SI thyristor 1b. The anode of the photo SI thyristor 1a is connected to a load 3 to which a voltage source E1 is connected, and the the cathode of the photo SI thyristor 1a is grounded. The gate of the photo SI thyristor 1a is connected to the anode of the photo SI thyristor 1b, the cathode of which is connected to a negative voltage source E2.
The photo SI thyristor 1a is turned on in response to the incidence of a trigger light LT, while the photo SI thyristor 1b is turned on in response to the incidence of a quenching light LQ. When the photo SI thyristor 1b is turned on, electric charges accumulated at the gate of the photo SI thyristor 1a are removed or drawn so as to turn off the photo SI thyristor 1a.
FIG. 16 is another example of a photo SI thyristor driving circuit. In this driving circuit, an electrolytic condensor C is connected between the gate of a photo SI thyristor 1a and the anode of a photo SI thyristor 1b, and the cathodes of the photo SI thyristors 1a and 1b are connected together and grounded. The anode of the photo SI thyristor 1b is connected to a common point of the cathodes of photo SI thyristors 5a and 5b connected in a darlington form, A resistor Rc is connected between the anode of photo SI thyristor 5a and the gate of the photo SI thyristor 5b.
A load 3 is connected between the anode of the photo SI thyristor 1a and a voltage source E1. A voltage source E3 is connected to the connecting point of the anode of the photo SI thyristor 5a and the resistor Rc.
As described above, the photo SI thyristor driving circuit of FIG. 15 must be provided with positive and negative voltage sources, so that there is a disadvantage in that the driving circuit is complex. In order to overcome this disadvantage, the photo SI thyristor driving circuit of FIG. 16 substitutes the electrolytic condensor C for the negative voltage source to generate a negative bias voltage, which is applied to the gate of the photo SI thyristor 1a to turn off the photo SI thyristor 1a. However, as the electrolytic condensor C must be installed at the outside of a package, a number of parts to be installed increases. Further as the positive voltage source E3 applied to the photo SI thyristors connected in the form of darlington must be provided, an improvement is required.
It is required that photo SI thyristors have a good switching characteristic if they are utilized as optical relays. However, in the photo SI thyristor driving circuits shown, in turning on the photo SI thyristors, charges must be accumulated at the gate thereof, while in turning off them, charges accumulated at the gate must be drawn. Accordingly, constituting the driving circuit with two stages of the photo SI thyristors for switching, as shown in FIG. 15, is a cause for delaying of switching performance of the photo SI thyristor 1a.
Further, it is necessary to provide the photo SI thyristor with a protection circuit against excess current because the photo SI thyristor element is possibly broken down due to heat occurring when excess current more than rated current of the element flows as a turning-on current.